Controlling the wettability of copper surfaces is of great interest for many applications in phase change heat transfer. The wettability of surfaces has been shown to have a prominent role on the nucleate pool boiling regime. In fact, hydrophobic surfaces provide a higher number of active nucleation sites which results in higher heat transfer coefficient (HTC) [references 1, 2] whereas hydrophilic surfaces delay the onset of the so-called “critical heat flux” (CHF) [references 3, 4] that is the maximum heat flux the surface can transfer to the liquid.
The modification of both the texture and chemistry of the surface can control the wettability of liquids on the surface.
Recent developments in micro and nanotechnologies have allowed engineers and scientists to fabricate surfaces that combine microscale and nanoscale features to address the multiple length scales associated with boiling, from nucleation (O(nm)), to bubble dynamics (O(cm)). Additionally, the surface chemistry can now be controlled with high surface energy coatings such as oxides or low surface energy coating such as silanes, non-polar carbon or fluorocarbons.
Many microfabrication techniques have been implemented in heat transfer manufacturing to fabricate multiscale surfaces. Liter and Kaviany [reference 5] have designed a porous multiscale surface of superimposed copper microspheres (200 μm diameter) to form an array of conical stacks, to delay the CHF onset. Another possibility of multiscale features fabrication has been discussed in [reference 6] by using the so-called “Microreactor Assisted Nanomaterial Deposition” (MAND) process. This “flower-like” texture deposited on aluminum and silicon enhanced both the HTC (10 times higher than bare surface) and the CHF (4 times higher). Recently, embossed fins (0.5-1 mm) combined with microscale cavities at the corner of the base and floor of the fin have been shown to drastically enhance the HTC (8 times higher than a plain copper surface) and the CHF (2.5 times higher) [reference 7].
One possible technique to fabricate surfaces with a high surface energy is to oxidize the surface. Liaw and Dhir [reference 8] synthetize an oxide on top of copper surfaces by heating the copper samples in air to obtain a thin layer of copper oxide. The resulting contact angle was measured to be in the range 75°-22° and its influence in pool boiling was studied. Later, Coursey and Kim [reference 9] studied the effect of wettability in nanofluids boiling and also oxidized copper by heating a sample in air. The process of heating copper in air results in the formation of a black copper oxide (II) CuO layer also known as cupric oxide [10]. This method has been investigated or used in many studies, e.g. [reference 11]. A detailed review has been written [reference 10] on the possibility to obtain copper oxide by heating method and chemical modification. In the case of chemical modification, it has been shown that alkali solutions yield to the formation of CuO (Copper oxide (I) also known as cuprous oxide).
Takata et al. [reference 12] deposited and coated copper surfaces with TiO2 layer (thickness 250 nm) to change the wettability of the surface. The same investigators also recently combined patterned biphilic surfaces on copper coated with the TiO2 layer for the hydrophilic area and with Teflon hydrophobic spots (diameter equal to or greater than 2 mm) [reference 13].
To obtain surfaces with low surface energy, investigators have used chemical modification involving bonding of two (2) hydroxyls groups to form copper hydroxide [reference 14]. However, no water pool boiling studies of copper hydroxide have been carried.
Others, such as Yao et al. [reference 15], have shown a possibility to form Cu(OH)2 nano needles from an ammonia solution. They reported a very high contact angles after additionally coating a FAS monolayer. A similar process to obtain microflowers was discussed in [reference 16]. Finally, a novel electro-deposition approach, based on an alkali solution, has been presented to create microstructure on an anodic copper plate [reference 17]. This method, combined with a fluoroalkasylane (FAS) monolayer coating, can result in high static contact angles (up to 165°) on copper with low hysteresis (down to 3°).
Other investigators have employed etching of the copper chemically using a solution of hydrochloric acid (HCl) and Acetic Acid also combined with a FAS coating can result in high contact angle (up to 153°), with low hysteresis (down to 10°) by modifying the surface texture randomly (etching) and the surface energy (coating) [reference 18]. Recently, etching techniques (Nitric acid assisted by cetyltrimethyl ammonium bromide and ultrasonication) combined with a silane coating were used to obtain superhydrophobicity on a randomly microstructured copper [reference 19].Finally, superhydrophobic surfaces)(CA=170°) were produced by the etching of polycristaline copper samples using etchants common in the microelectronics industry (by electrodeposition of copper films with subsequent nanowire decoration based on thermal oxidation) [reference 20].
However, the fabrication of copper biphilic surfaces for pool boiling is challenging regarding the (i) chemical stability for several superhydrophilic patterning techniques, as is the case with UV-irradiation of TiO2 containing coatings which revert to their original hydrophobic state when stored [reference 25] and (ii) thermal stability for surfaces and coatings in phase change heat transfer. Additionally, micro and nanostructures may exhibit different thermal behavior compared to bulk materials [reference 26].
The possibilities to control the texture and chemistry of a surface are summarized in FIG. 1 below. On this figure, the resulting contact angles are also mentioned. The surface engineering and the contact angle references are compared to those achieved by the present invention.
Several functional surfaces able to reversibly switch their wettability have been described since the beginning of this millennium, mostly with polymeric materials. These surfaces can be classified according to the stimulus used [reference 50]. An electrically responsive surface was developed on a carboxylate-terminated self-assembled monolayer undergoing conformational transitions resulting in a reversible change of wettability properties of the surface, from a hydrophilic to a less hydrophilic wetting state (the static contact angle could be tuned in the range θ* about 25° to θ* about 45°) [reference 51]. The surface doping of a polypyrrole (PPy) conducting polymer can be controlled by applying a voltage to tune the static contact angle from θ* about 0° to θ* about 152° on the PFOS-doped (oxidized) PPy and dedoped (neutral) PPy films respectively [reference 52]. Electrowetting, an electrical stimulus, was shown to dynamically control the wetting behavior of liquids (with contact angle ranges from θ* about 180° to θ* about 0°) on a nanostructured silicon surface coated with a fluorocarbon polymer [reference 53]. A thermally responsive functional surface has been designed on a poly (N-isopropylacrylamide, i.e. PNIPAAm)-modified surface. This surface was shown to adapt its wettability from a static contact angle value θ* about 0° at a temperature T<29° C. to θ* about 149° at T>40° C. by reversible formation of intermolecular hydrogen bonds between PNIPAAm chains and water molecules [reference 54]. A thermal stimulus was also applied to a superhydrophobic surface consisting of arrays of micro-pillars fabricated with a liquid crystal elastomer, and resulted in the deformation of the pillars for a precise control of the wettability [reference 61]. Surfaces responsive to pH changes have also been reported [reference 55]: a monolayer containing both alkyl and carboxylic functional groups was formed on a gold-coated surface to reversibly switch from superhydrophobicity (θ* about 154°) for acid (pH 1) droplets to superhydrophilicity (θ* about 0°) for base (pH 13) droplets.
On metallic surfaces, functional wettability change has also been achieved. Photo-responsive surfaces were obtained on metal oxide materials such as TiO2, ZnO, SnO2 and V2O5 [references 50, 56, 57]. For instance, V2O5 nanostructured surfaces were shown to reversibly transition from a superhydrophilic wetting state (θ* about 0°) to a superhydrophobic wetting state (θ* about 160°) by exposure to UV light and maintenance in a dark storage environment, respectively [reference 56].
Applications of these surfaces to phase change heat transfer is currently very limited. The only recent example was with a silicon surface coated with gold electrodes to control boiling spatially on the scale of a few millimeters and temporally in the subsecond range [reference 58]. In this precursor work on the control of nucleation, the wettability could also be controlled with electrical pulses causing a limited variation of the contact angle from 75° to 100°. Also, the performance of silicon in pool boiling is inferior to the one of copper because of a lower thermal conductivity (1.3 W/cm·K for silicon compared to 385 W/cm·K for copper), which is a limiting factor for industrial applications. Furthermore, polymer surfaces or thin coatings might be damaged by the shear or thermal stresses [reference 42] associated with phase change heat transfer. Some functional coatings are also limited to specific applications: functional surfaces triggered by electrowetting are more suitable to act on a single, individual droplet and require the use of electrodes limiting their integration into channel configurations. The lithographic exposure required in photo-responsive materials is not convenient for industrial applications, especially at a large scale.